1. Field of Invention
The present invention, in general, relates to methods for the formation of optical crystal layers and, in particular, to methods for the formation of thin optical crystal layers on a substrate.
2. Description of the Related Art
Conventional electro-optical devices (e.g., optical switches, optical modulators, optical polarizers, optical amplifiers, optical lasers, etc.) are manufactured using bulk optical crystal layers (e.g., a lithium niobate [LiNbO3] single crystal wafer) with a thickness of at least 1 mm overlying a substrate. The manufacturing of such conventional electro-optical devices often includes forming waveguide regions in the bulk optical crystal layer using titanium diffusion or proton exchange and coating the bulk optical crystal layer with a dielectric buffer layer (e.g., a silicon dioxide [SiO2] buffer layer). Although they have become widely accepted, the conventional electro-optical devices can suffer such drawbacks as large Radio-Frequency (RF) and optical wave velocity mismatch and the inability to provide a broad bandwidth at a low modulation or switching voltage.
Various thin ferroelectric optical crystal layer preparation methods, including RF-sputtering, ion beam sputtering, pulsed laser deposition, metal-organic Chemical Vapor Deposition (CVD), Plasma-Enhanced CVD, epitaxial and sol-gel methods, have been investigated as alternatives to the use of bulk optical crystal substrates in electro-optical device manufacturing. However, each of these methods has been ineffective in producing a thin optical crystal layer with the electro-optical coefficient, surface quality, homogeneity and single crystal structure required for use in electro-optical devices.
In the field of semiconductor manufacturing, methods are known for the formation of a thin single crystal silicon layer on a silicon dioxide layer overlying a silicon wafer. These methods involve implanting ions into a silicon substrate to define a thin single crystal silicon layer thereon. The thin single crystal silicon layer is then bonded to a silicon dioxide layer overlying a silicon wafer. A thin single crystal silicon layer, silicon dioxide layer and silicon wafer structure is subsequently separated from the silicon substrate with a high temperature anneal. Such methods, however, are not suitable for the formation of a thin optical crystal layer due to the required high bond strength of optical crystal materials. In addition, the high temperature anneal is incompatible with forming a thin optical crystal layer on a substrate with mismatched thermal expansion characteristics.
Still needed in the field of electro-optical devices, therefore, is a method for the formation of a thin optical crystal layer with characteristics (e.g., electro-optical coefficient, surface quality, homogeneity and single crystal structure) that are suitable for use in electro-optical devices. In addition, the method should form the thin optical crystal layer in a configuration that enables the manufacturing of electro-optical devices with a reduced RF and optical wave velocity mismatch, a broad bandwidth and a low modulation or switching voltage.
The present invention provides a method for the formation of a thin optical crystal layer with characteristics that are suitable for use in, and in a configuration that enables the manufacturing of, electro-optical devices with a reduced RF and optical wave velocity mismatch, a broad bandwidth and a low modulation or switching voltage.
A method according to the present invention forms a thin optical crystal layer (e.g., a thin LiNbO3 optical single crystal layer) overlying a low dielectric constant substrate (e.g., a low dielectric constant glass substrate). The thin optical crystal layer has characteristics (e.g., electro-optical coefficient, surface quality, homogeneity, single crystal structure, etc.) that are equivalent to a bulk optical crystal and is, thus, suitable for use in electro-optical devices. Furthermore, the method forms the thin optical crystal layer in a configuration (i.e., overlying a low dielectric constant substrate) that enables the manufacturing of electro-optical devices with a reduced RF and optical wave velocity mismatch, a broad bandwidth and a low modulation or switching voltage.
Processes in accordance with the present invention include first implanting ions (e.g., He+ ions) into an optical crystal substrate (e.g., a LiNbO3 optical single crystal wafer). The implanted ions define, in the optical crystal substrate, a thin ion-implanted optical crystal layer overlying a bulk optical crystal substrate. A low dielectric constant substrate is subsequently bonded to the optical crystal substrate, using either a direct or an indirect bonding technique, to form a bonded structure. The bonded structure is thermally annealed at a temperature in the range of 300xc2x0 C. to 600xc2x0 C. for 30 minutes to 300 minutes. The thin ion-implanted optical crystal layer and the low dielectric constant substrate are subsequently separated from the bulk optical crystal substrate by applying mechanical force to the low dielectric constant substrate and/or bulk optical crystal substrate in the direction of separation. This separation results in the formation of a thin optical crystal layer overlying a low dielectric constant substrate.
A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description that sets forth illustrative embodiments, in which the principles of the invention are utilized, and the accompanying drawings.